Ethylene oxide (EO)-capped polyols are valuable in the polyurethane industry because the primary hydroxyl groups of EO-capped polyols react favorably with polyisocyanates. Ethylene oxide-capped polyols are typically produced by a multi-step process. First, propylene oxide (PO) (or a mixture of PO and EO) is polymerized in the presence of a basic catalyst (usually potassium hydroxide (KOH)) to produce a polyol containing mostly secondary hydroxyl groups. In the second step, EO is added to the catalyst-containing mixture to convert some or most of the secondary hydroxyl groups to primary hydroxyl groups. The typical process uses the same catalyst (usually KOH) for both propoxylation and ethoxylation. After the addition of EO is complete, the basic catalyst is either neutralized with an acid and the precipitated salt is separated from the polyol by filtration or centrifugation, or the basic catalyst is removed with an ion exchanger, coalescer, absorbent or any of the other techniques known in the art to produce a neutralized polyol.
DMC catalysts can be used to produce polyether, polyester and polyetherester polyols. These polyols may be used to produce polyurethane coatings, elastomers, sealants foams, adhesives and the like. DMC catalysts, such as zinc hexacyanocobaltate, offer many advantages in the production of polyether polyols. For example, DMC catalysts can be used to produce polyether polyols that have intrinsically low unsaturation levels compared to polyether polyols produced by basic (KOH) catalysis.
The various advantages of using low unsaturation polyols in the production of polyurethanes have been described in the following disclosures: EP 0 876 416; U.S. Pat. No. 5,700,847; and WO 99/51657. Improvements in DMC catalyst technology have provided catalysts with increased activity for epoxide polymerization. See, e.g., U.S. Pat. Nos. 5,470,813; 5,482,908; 5,545,601; and 5,714,428.
Despite the many advantages of using DMC catalysts in the production of polyols, one important drawback remains, that is, DMC catalysts are inefficient at adding oxyethylene groups to high equivalent weight polyols for the purpose of raising the average primary hydroxyl content. Ethylene oxide cannot be added to “cap” an oxypropylene polyol prepared by DMC catalysis, as is done in KOH catalysis. As the endgroup concentration becomes critically low, at hydroxyl numbers below about 50 mg KOH/g, additional oxyethylene preferentially adds to existing primary groups and a good distribution is not achieved. It therefore becomes impractical to utilize DMC catalysis for adding oxyethylene for the purpose of raising primary hydroxyl content. Where EO is added to a high equivalent weight polyoxypropylene polyol produced by DMC catalysis, the resulting product is a heterogeneous mixture of: (1) unreacted polyoxypropylene polyol; and (2) highly ethoxylated polyoxy-propylene polyol and/or polyethylene oxide. As a result, the product produced by an EO-capped polyoxypropylene polyol, which was produced by DMC catalysis, is hazy and, at times, solid at room temperature.
Several different processes have been developed in attempting to overcome this drawback. These processes involve preparing an EO-capped polyol from a DMC-catalyzed polyol with “re-catalysis”. Re-catalysis involves preparing an oxypropylene polyol by DMC catalysis, adding a basic catalyst to the DMC-catalyzed oxypropylene polyol and then adding EO to cap the polyol.
For example, U.S. Pat. No. 4,355,188 discloses a process that involves capping a DMC-catalyzed polyol with EO while the polyol is in contact with a strong base. The strong base is removed from the polyol after EO capping is complete. The work-up of this polyol can be accomplished by neutralization of the strong base with a strong acid, for example sulfuric or phosphoric acid, as well as separation of the precipitated salt by filtration or centrifugation. If the precipitated salt is allowed to remain in the polyol, blockages in foaming equipment will result. Additionally, precipitated salts remaining in the polyol can adversely impact the physical properties of the polyol.
The use of ion exchangers provides another potential method of removing the basic catalyst following the production of EO-capped polyols via the re-catalysis method. (See Kirk-Othmer, Encyclopedia Of Chemical Technology, 2nd Ed., Vol. 11, 1966, Interscience Publishers, New York, pages 871 to 899.)
EO capping by a re-catalysis approach thus imposes additional processing costs from several factors. The addition of the new catalyst and its preparation must be effectively managed, either by adding directly or through a heel. But any re-catalysis adds an additional processing step that incurs increased manufacturing cost and decreased efficiency. The requirement of a refining step for removing the basic catalyst adds another process and associated manufacturing costs.
An additional and very significant drawback to the re-catalysis method lies in the requirement of two reactors instead of one. As those skilled in the art are aware, basic catalysts act as poisons to DMC catalysts. The typical re-catalysis manufacturing method is therefore to synthesize the DMC-catalyzed intermediate in one reactor and add the base-catalyzed EO cap in a second, different reactor. This two-reactor method decreases efficiency in the manufacturing process.
Japanese Kokai H5-25267 discloses a process in which re-catalysis is carried out with an aqueous solution of KOH. Following the addition of an aqueous solution of KOH, but before the addition of a certain amount of monoepoxide having 3 or more carbon atoms, water is removed to a certain level. EO is added to convert secondary hydroxyl groups to primary hydroxyl groups. However, to remove the added catalyst, work-up of the polyol is necessary after EO-capping.
U.S. Pat. No. 5,144,093 discloses a process in which a DMC catalyst residue-containing polyol is reacted with an oxidant to cause the catalyst residue to form insoluble residues. The insoluble residues are separated from the polyol to produce a polyol which is essentially free of DMC catalyst residues. The insoluble residues are separated from the polyol before it is treated with a base to provide a base-treated polyol that is reacted with EO to produce an EO-capped polyol.
A process for preparing EO-capped polyols from DMC-catalyzed polyols without using re-catalysis is disclosed in U.S. Pat. No. 5,563,221. The '221 patent teaches a first polyol prepared with a DMC catalyst blended with a second polyol prepared with a basic catalyst, in which the basic catalyst is present in an amount from 0.05 wt. % to about 2 wt. %, based on the total weight of the polyol blend. The polyol blend is reacted with EO to produce an EO-capped polyol. The basic catalyst is present in a concentration that allows for deactivation of the DMC catalyst as well for catalyzing ethoxylation of the polyol blend. Following ethoxylation, the EO-capped polyol is purified to remove catalyst residues.
Thus, while the art discloses processes for producing EO-capped polyols in the presence of DMC catalysts, these teachings all require the removal of catalyst residue(s) or salt(s) formed by the neutralization of a basic catalyst. Moreover, the art does not provide for neutralizing residue (i.e., the “heel”) from the base-catalyzed capping step of preceding polyol batches in the reactor used for DMC synthesis. It is assumed that either sufficiently rigorous cleaning of the reactor has taken place between batches or that a two reactor process has been used. Either of those approaches adds processing costs and decreases efficiency.
In U.S. Pat. No. 6,077,978, the starting mixture (referred to as the “heel”) is acidified to neutralize residual alkalinity in the glycerin during the subsequent continuous-addition-of-starter (CAOS) feed or basic impurities in the actual starter itself. The “heel” in the '978 patent is the entire starting charge and is acidic and the goal is prevention of DMC catalyst deactivation in an all-DMC reactor.
The art also discloses a process for neutralizing polyether polyols produced by basic catalysis. U.S. Pat. No. 4,110,268 discloses neutralizing, with dodecylbenzene sulfonic acid (DDBSA), a polyether polyol prepared by basic catalysis. This neutralization step results in the reduction or elimination of purification procedures. The '268 patent, however, is directed to producing polyether polyols by basic catalysis, without using “extraneous” catalysts. The '268 patent also discloses that even when “extraneous” catalysts are required in the polyol foam formulation, “very substantially” reduced amounts of the “extraneous” catalysts are used. The use of some soluble acids such as DDBSA to neutralize conventional levels of basic catalysts (e.g. 0.1–1 wt. % KOH based on the final product) can create an additional problem in that some applications (i.e., flexible foam production) have extreme sensitivity to the presence of surface active agents, such as the salt of DDBSA. The '268 patent does not disclose a method for minimizing the presence of such byproducts.
Therefore, a need exists in the art for a process for ethylene oxide (EO)-capping double metal cyanide (DMC)-catalyzed intermediates within a single reactor. It is furthermore desirable to develop a process that minimizes the presence of surface-active byproducts in the final polyol product.